NAIROBI, Kenya — Sudan sent military aircraft over a key South Sudanese city Tuesday as part of a two-day bombing campaign that has targeted South Sudanese military positions along the two nations' disputed border.

No explosives fell from the aircraft, a pair of Russian-made Antonov cargo planes that Sudan has long used to drop bombs on opposing forces, as they "hovered" above Bentiu, the capital of South Sudan's Unity state.

But their presence, just a week before Sudanese President Omar Bashir was to visit South Sudan's capital, Juba, for a presidential peace summit, was a reminder of how intractable tensions between the two countries have become. Some Sudanese media reported that Bashir's visit has been canceled.

On Monday, South Sudan's president, Salva Kiir, warned that his country was being pushed toward war as border skirmishes over the oil-rich Heglig region, which both countries claim, have turned into fierce combat this week between the nations' armies.

"I said many times that I don't want to bring us back to war. But they (Sudan) are looking for it," Kiir told members of his political party in response to the fighting.

Sudanese army and militia forces attacked South Sudanese forces near the border on Monday, according to information the United Nations received from its local security adviser. Sudanese aircraft also bombed South Sudanese positions, the U.N. confirmed. The U.N. said that 10 wounded South Sudan soldiers were brought to the Bentiu hospital from the front lines.

According to the South Sudanese spokesman, Philip Aguer, the two sides' armies were positioned just 500 meters from one another, awaiting a long-delayed border demarcation team, when the Sudanese attacked.

"We were told by the government to wait for the commission to come and tell us who is on the right side and who is on the wrong side," he said.

The overflight of Bentiu also was a reminder that Sudan's aircraft go virtually unchallenged over South Sudan, which has no fixed-wing military planes and whose handful of MI-17 transport helicopters for the most part are inoperable.

There were conflicting reports about who controlled what as the fighting continued Tuesday.

A spokesman for the Sudanese military did not respond to several calls to his cellphone throughout the day.

How seriously to take the clashes was unclear. Some analysts noted that the two nations' armies have clashed several times since South Sudan became independent from Sudan last summer. "Given the picture of underlying incentives for cooperation, they can still pull back from incidents like this, and that's what I would expect still to happen," said Richard Barltrop, an independent Sudan researcher and author.

The fighting was discussed by the U.N. Security Council in New York, however, and White House officials expressed alarm at the developments.

"Both sides must exert the greatest restraint in this situation," the White House said in a statement that urged Bashir and Kir not to cancel their summit meeting scheduled for next Tuesday.

"Only through direct contact and negotiations over fundamental issues of security and border management ... can Sudan and South Sudan avoid further fighting, achieve vitally needed economic cooperation, and coexist in peace," the statement said.

Besides the disputed demarcation line in the Heglig region, Sudan and South Sudan are believed to be supporting rebel forces in their rivals' territory.

The fighting in Sudan's Nuba Mountains, where South Sudan is believed to be supporting old friends-in-arms, in particular has sparked concerns of a humanitarian disaster. Sudanese aircraft regularly bomb villages, and the coming harvest is expected to be much less than normal. The United States is mulling a plan to send food aid directly across the border to avert famine in the coming months if the Sudanese government continues to block humanitarian access.

The road that humanitarian aid would travel is the same road as that is used to supply the Nuba rebels. There has been fierce fighting over control of that road, and Sudan could be trying to block it before the rainy season begins in April and ground offensives become difficult.

Arnu Ngutulu, a spokesman for the Nuba rebels, said they were not involved in the recent fighting. "I'm just hearing about it from the news, too," he said by phone.

One hopeful sign that hostilities might be avoided was that the South Sudanese president did not blame his Sudanese counterpart for the fighting, but instead singled out unnamed hard-line elements trying to sabotage Bashir's visit.

"These are people of course who don't want Bashir's visit," Kiir said in an Arabic aside on Monday in an audio recording of his remarks obtained by McClatchy. "These are the people who are causing this fighting."

(Boswell is a McClatchy special correspondent. His reporting is underwritten in part by a grant from Humanity United, a California-based foundation that focuses on human rights issues.)

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